First Application in Oman of New Single Stage Retarded Sandstone Matrix Acidizing

Abstract

The objective of matrix acidizing in sandstone reservoirs using acid systems that contains Hydrofluoric acid (HF) is to widen the pore throats and spaces in order to increase the permeability around the wellbore and also to remove formation damage. One of the major disadvantages of this acid system is the secondary and tertiary reactions, which may end up with precipitations that damage the formation. Because of this, pumping sufficient pre- flush and post-flush volumes of Hydrochloric acid (HCl) is critical to prevent such damaging reactions. However, the placement of such fluids still are a concern in multiple opened layers or long open intervals zones. Stimulating sandstone reservoirs in the Southern fields of the Sultanate of Oman is very challenging, especially in those that exhibit relatively low permeability. These formations, based on petrology work, contains significant amount of clays and feldspars, which make it difficult in the designing process of the acid formulation. A new version of HF acid system was recently developed. It is specially formulated, so it does not require the addition of Hydrochloric acid (HCl) pre-flush. Because of this, it can be pumped as a single stage system. In addition, its higher reactivity allows deeper penetration and it has the ability to minimize secondary reactions and damaging precipitates. Lab testing work was conducted to ensure the effectiveness of this single stage acid system. The results were promising as they show a good improvement in the rock permeability. These results were encouraging to carry field trials in the sandstone reservoirs in Oman Southern fields. Up to now, it has been pumped in these type of sandstones for oil producer wells and for water injector wells. The actual treatment using this system showed increased oil productivity by higher than 60% and higher than 80% in water injectivity. This paper presents the testing, designing and pumping of the single stage acid system, as well as the comparison with the conventional HF acid system in Southern fields of Oman. It outlines the laboratory work and analysis done as well as the field trials.